The field of the invention relates to air/fuel control systems for internal combustion engines equipped with catalytic converters.
It is known to correct fuel delivered to an internal combustion engine in response to feedback from a two-state (rich/lean) exhaust gas oxygen sensor. A proportional plus integral feedback controller responsive to the sensor output will force the engine's operating air/fuel ratio to oscillate or hunt around the sensor switch point. If the sensor switch point actually corresponds to stoichiometric combustion, the efficiency of a three-way catalytic converter (NOx, CO, and HC) will be optimized.
Feedback systems are also known in which the exhaust sensor is placed downstream of the catalyst where exhaust gases are near equilibrium and average engine air/fuel operation is more likely to average at stoichiometry.
The inventors herein have recognized that the operating window for maximum efficiency of a catalytic converter does not always correspond to the switch point the oxygen gas sensor used in a feedback control system. Even when a relatively good correspondence is initially achieved, aging and temperature effects of the oxygen sensor may cause a variance between the sensor indication and the actual conversion window of the catalytic converter.